(a) Technical Field
The present invention relates to a fuel cell stack. More particularly, the present invention relates to a fuel cell stack which employs a separator, in which two or more reaction areas are connected in an insulated manner to maintain the output voltage at a high level and the output current at a low level under the same output of the fuel cell stack, and improve the productivity of the fuel cell stack, thus reducing the manufacturing cost.
(b) Background Art
The configuration of a unit cell of a fuel cell stack will be described with reference to FIGS. 6 and 7. A membrane electrolyte assembly (MEA) is positioned in the center of the unit cell. The membrane electrolyte assembly includes a polymer electrolyte membrane 11 capable of transporting hydrogen ions (protons) and catalyst layers such as a cathode 12 and an anode 13, which are coated on both sides of the electrolyte membrane 11 such that hydrogen and oxygen react with each other.
Moreover, a gas diffusion layer (GDL) 16 and a gasket 18 are sequentially stacked on the outside of each of the cathode 12 and the anode 13, a separator 20 in which flow fields are formed to supply and discharge a fuel (hydrogen) and an oxidant (air) and discharge water produced by a reaction is stacked on the outside of the gas diffusion layer 16, and an end plate 30 for supporting and fixing the above-described components is connected to the outermost end.
Accordingly, at the anode 13 of the fuel cell stack, an oxidation reaction of hydrogen occurs to produce hydrogen ions (protons) and electrons, and the produced hydrogen ions and electrons are transmitted to the cathode 12 through the electrolyte membrane 10 and the separator 20. At the cathode 12, the hydrogen ions and electrons transmitted from the anode 13 react with the oxygen-containing air to produce water. At the same time, electrical energy is generated by the flow of electrons and supplied to a load requiring the electrical energy through a current collector connected to the end plate 30.
Meanwhile, as shown in FIGS. 6 and 7, the separator 20 includes a reaction area 23 consisting of flat lands 21 that are in direct contact with the gas diffusion layer 16 and channels 22 each located between the lands 21 and serving as a passage of the fuel such as hydrogen and air (oxygen) and a manifold configured in the form of a through aperture for supply and discharge of hydrogen, air, coolant, etc. and provided on both ends of the reaction area 23.
Here, the manifold consists of an air supply manifold 24, a coolant supply manifold 25, and a hydrogen supply manifold 26, which are formed in parallel on one end of the reaction area 23, and an air discharge manifold 27, a coolant discharge manifold 28, and a hydrogen discharge manifold 29, which are formed in parallel on the other end of the reaction area 2.
For reference, in a fuel cell stack shown in FIG. 6 in which the number of separators having a reaction area of 2A is n, and in a fuel cell stack shown in FIG. 7 in which the number of separators having a reaction area of A is 2n, the entire reaction areas of the two fuel cell stacks are the same, which represents that the outputs of the fuel cell stacks are the same, but the output voltages and currents of the fuel cell stacks are different. As such the Equation 1 below is applied:Fuel cell stack output=Reaction area(cm2)×Current density(A/cm2)×Cell number×Cell average voltage(V_cell).  [Equation 1]
The conventional separator 20 shown in the right of FIG. 6 has a large reaction area and is assembled in a fuel cell stack of a single module design. Since the reaction area (proportional to the current) is large and the number of cells (proportional to the voltage) is small, the voltage is relatively low and the current is high.
Due to these features, it is possible to facilitate the assembly of the fuel cell stack, reduce the size of the fuel cell stack, and the production costs associated therewith. However, since the voltage is relatively low, a high current is typically used, which results in a reduction in efficiency of a drive unit (e.g., a drive motor, an inverter, etc.) and thus causes a cooling problem, and in the case of the same output, the weight and volume of the drive unit increases, which is very problematic.
The conventional separator 20 shown in the right portion of FIG. 7 has a structure in which the reaction area is reduced to half of that shown in FIG. 6 and is assembled in a fuel cell stack of a dual module type in which two single modules are arranged up and down. Accordingly, it is possible to maintain the output voltage of the fuel cell stack at a high level (two times that of the fuel cell stack employing the separator of FIG. 6), maintain the output current at a low level (½ times that of the fuel cell stack employing the separator of FIG. 6), and improve the efficiency of the drive unit (e.g., a drive motor, an inverter, etc.). However, as the number of cells stacked increases, the number of manufacturing processes and parts increases, and thus the manufacturing cost increases.
In order to reduce the manufacturing costs and simplify the manufacturing process, the number of cells of the fuel cell stack including the above-described separator tends to be reduced, and the reaction area of the separator tends to be increased. As such, when the number of cells of the fuel cell stack is reduced and the reaction area of the separator is increased, when the electrical power generated by the fuel cell stack is used by electrically powered components (e.g., a drive motor for driving a fuel cell stack, an inverter, etc.) at the same output, it is advantageous to maintain the voltage generated by the fuel cell stack at a high level and the current at a low level, and thus generally the output of the fuel stack is often boosted to increase the voltage and reduce the current.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.